An electric power steering apparatus (EPS) which provides a steering mechanism of a vehicle with a steering assist torque by means of a rotational torque of a motor, applies a driving force of the motor that is controlled by electric power supplied from an inverter as the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears. In order to accurately generate the steering assist torque, such a conventional electric power steering apparatus performs feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of duty command values of pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a steering wheel 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack-and-pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1 and a steering angle sensor 14 for detecting a steering angle θ, and a motor 20 for assisting a steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. The electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist control on the basis of a steering torque Th detected by the torque sensor 10 and a vehicle speed V detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 for the EPS on the basis of a voltage control command value Vref obtained by performing compensation or the like with respect to the current command value.
Moreover, the steering angle sensor 14 is not essential, it does not need to be provided, and it is possible to obtain the steering angle from a rotation sensor such as a resolver connected to the motor 20. It is possible to provide an auto-cruise switch 15, and input an auto-cruise signal AS.
A controller area network (CAN) 50 exchanging various information of a vehicle is connected to the control unit 30, and it is possible to receive the vehicle speed V from the CAN 50. Further, it is also possible to connect a non-CAN 51 exchanging a communication, analog/digital signals, a radio wave or the like except with the CAN to the control unit 30.
The control unit 30 mainly comprises a CPU (also including an MPU, an MCU or the like), and general functions performed by programs within the CPU are shown in FIG. 2.
The control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Th detected by the torque sensor 10 and the vehicle speed V detected by the vehicle speed sensor 12 (or sent from the CAN 50) are inputted into a current command value calculating section 31 for calculating a current command value Iref1. The current command value calculating section 31 calculates the current command value Iref1 that is a control target value of a current supplied to the motor 20 on the basis of the inputted steering torque Th and the inputted vehicle speed V and by using an assist map or the like. The current command value Iref1 is inputted into a current limiting section 33 through an adding section 32A. A current command value Irefm of which a maximum current is limited is inputted into a subtracting section 32B, and a deviation I (Irefm−Im) between the current command value Irefm and a motor current value Im is calculated. The deviation I is inputted into a PI control section 35 for characteristic improvement of steering operations. The voltage control command value Vref of which the characteristic is improved by the PI control section 35 is inputted into a PWM control section 36. Furthermore, the motor 20 is PWM-driven through an inverter circuit 37. The current value Im of the motor 20 is detected by a motor current detector 38 and is fed back to the subtracting section 32B. The inverter circuit 37 is comprised of a bridge circuit of FETs which are used as driving elements.
Further, a compensation signal CM from a compensation signal generating section 34 is added in the adding section 32A, and characteristic compensation of the steering system is performed by the addition of the compensation signal CM so as to improve a convergence, an inertia characteristic or the like. The compensation signal generating section 34 adds a self-aligning torque (SAT) 343 and an inertia 342 in an adding section 344, further, adds the result of addition performed in the adding section 344 and a convergence 341 in an adding section 345, and then outputs the result of addition performed in the adding section 345 as the compensation signal CM.
In such the electric power steering apparatus, a paved surface of a roadway is sloped about 1-2% from a center line to a road shoulder for the purpose of drainage or the like, so that a vehicle tends to be flowed in the direction of the road shoulder, which is a lateral flow or a one-sided flow, if a steering wheel is not kept turned for a long time in the case of running on a straight road fast. Further, wheel alignment breaks because of aging of the vehicle (for example, settling of a suspension bush and aging of a vehicle body), collision to a curbstone or the like, so that the vehicle may not run straight, which is the lateral flow or the one-sided flow, if a driver does not give power to a steering wheel. Such running may impose a large burden to the driver.
Therefore, many improvement methods have been conventionally proposed. For example, a method shown in Japanese Published Unexamined Patent Application No. 2007-22169 A (Patent Document 1) estimates a slope of a road surface in accordance with a vehicle speed, a lateral acceleration (a lateral G), a steering state and information of a navigation system, and corrects a lateral flow or a one-sided flow of a vehicle. A method shown in Japanese Published Unexamined Patent Application No. 2008-207775 A (Patent Document 2) calculates a smoothed torque Ts1 by smoothing a short-term steering torque in straight running, calculates a smoothed torque Ts2 by smoothing a long-term steering torque, judges running on a cant (a slope of a road surface) by the relation between the smoothed torque Ts1 and the smoothed torque Ts2, and corrects a lateral flow or a one-sided flow of a vehicle.